1. Field Of The Invention
This invention relates to polyurethane elastomers having improved combustion resistance and to a method of preparing such elastomers.
2. Description Of The Prior Art
Urethane elastomers are a class of polymers which can be broadly defined as containing a relatively high molecular weight (i.e., &gt;1000) polyol (or polymer/polyol), a low molecular weight chain extender and an isocyanate. Such elastomers can be fabricated by reaction injection molding (commonly termed "RIM") techniques. RIM polymers are based upon a distribution of hard and soft segments. The process for preparing these polymers is described in Rubber Age, volume 7, page 46, 1975. The hard segments (i.e., the reaction product of the isocyanate and low molecular weight extenders) provide the modulus requirements, while the soft segments (i.e., the polyol) provide the resiliency or flexibility characteristics. The respective materials or monomers employed contain the reactive moieties at or near the extreme ends of the molecule. Upon reaction, the molecules are linked end-to-end in a chain fashion.
Elastomeric polymers ranging from very low to very high modulus can be formulated by use of the wide variety of intermediates available and by controlling the ratio of polyol to chain extender plus isocyanate. Because of their excellent properties and performance characteristics, high modulus polyurethane elastomers are of particular interest for numerous applications including those relating to automotive fascia, structural enclosures for the electronics industry, appliance housings and the like. Unfilled elastomers typically used for automotive facia have a modulus up to about 80,000 psi or so. Elastomers for structural applications preferably have somewhat higher modulus, such as, for example, on the order of about 100,000 psi. The modulus may of course be significantly higher, on the order of 160,000 psi to about 200,000 psi or 240,000 psi or so and higher, depending on the particular application.
The use of reaction injection molded thermoset polyurethane elastomers provides certain advantages over injection molding of thermoplastic resins. For example, the thermoplastic injection molding process typically requires that the thermoplastic resin employed be melted and then injected into a mold. In order to insure that the void spaces throughout the mold are filled, the resin must be heated to a temperature sufficiently high to reduce its viscosity and thereby permit the resin to be injected into the mold. Even when this is done, it is usually necessary to employ high pressure molding equipment to inject the resin into the mold to produce items of satisfactory marketable quality. In contrast, the oligomeric components of the RIM system are of relatively lower molecular weight and viscosity and need not be heated to the same degree as thermoplastics prior to injection into a mold. Because the RIM components have low viscosity when injected and react in situ in the mold, the RIM polyurethane technique more faithfully fills the mold and reproduces mold detail better than injection molding with thermoplastic materials.
A further advantage of RIM polyurethane production is that capital costs associated with RIM equipment are lower than those associated with comparable thermoplastic molding equipment because the need for high pressure equipment required by the thermoplastic injection process is reduced. Similarly, the energy costs associated with the RIM process are much lower than those associated with thermoplastic molding because the degree of mold heating and cooling is much less than with thermoplastic molding equipment.
Despite these several processing and performance advantages of RIM urethane elastomers, such elastomers have not been commercially successful for many consumer and industrial applications for which flammability and combustion resistance standards are stringent. Such applications include appliance housings, such as, for example, air conditioners, refrigerators and the like, and for electronics applications such as, for example, housings for computers and the like.
Industry recognized combustion resistance and flammability requirements for plastic materials in general, including polyurethanes which are utilized in such applications, are extremely demanding. Such materials are subjected to and must pass a vertical burn test such as the vertical burn test established by Underwriter's Laboratories, Inc. which is well known to those skilled in the art. Materials are rated according to the requirements established by that test depending upon certain characteristics exhibited by the material when it is subjected to the test, including the ability of the plastic not to drip flaming particles and the ability of the plastic to burn at a slow rate or to self-extinguish. In order to qualify for certain electrical and electronic enclosure applications, it is necessary for the plastic material to exhibit a relatively short burn time and/or to be self-extinguishing within certain periods of time, and for certain ratings it must not drip flaming particles. Such enclosures, as well as other applications, represent a large potential market for RIM polyurethanes. However, certain RIM polyurethanes have not been commercially feasible for such applications because of their tendency to burn rapidly once ignited and to drip flaming particles.
While material density and the thickness of manufactured parts can be adjusted to improve flammability characteristics of polyurethane products, more significant improvements in flammability performance have been achieved by the incorporation of flame retardant additives. Significant reduction in the flammability of polyurethane compositions has been achieved by incorporation of known halogen and phosphorus flame retardant additives. Illustrative of the approach are:
Japanese Pat. No. 58,017,116 which discloses articles manufactured from a fire-retardant hard urethane usable as housings for various electrical applicances and household equipment; the articles are produced from a process mixing components comprising a foam stabilizer, polyisocyanate and a liquid mixture of active hydrogen compounds.
U.S. Pat. No. 3,849,368 which discloses polymer compositions with flame-retardant properties comprising thermally stable, cyclic phosphonate esters.
European Pat. No. 62,210 which discloses polyurethane elastomer compositions with flame-retardant properties comprising at least four additives comprising Sb.sub.2 O.sub.3, halogen compounds, alumina trihydrate, phosphate triester and optionally quaternary tetraalkyl ammonium salts.
U.S. Pat. No. 4,407,981 which discloses polyurethane compositions with flame-retardant properties comprising a dialkyl alkylphosphate and an organochlorine or organobromine compound.
U.S. Pat. No. 3,966,478 which discloses polyurethane compositions with flame-retardant properties comprising a haloalkyl phosphoramidate flame-retardant additive.
U.S. Pat. No. 4,273,881 which discloses polyurethane foams with flame-retardant properties; chlorinated phosphorus compounds such as bis-(2-chloroethyl)-2-chloroethyl phosphonate are incorporated as flame-retardant additives.
Approaches to reduce dripping and impart flame-retardancy include:
U.S. Pat. No. 4,342,682 which discloses polyurethane elastomers with flame-retardant and anti-drip properties comprising melammonium pentate and/or a pentate salt of ammelide as a bubbling agent to form a non-burning char.
U.S. Pat. No. 4,381,364 which discloses flame-retardant polyurethanes which produce char and fail to drip on combustion; the composition comprises a thermoplastic polyurethane, polyvinyl chloride, a polyacrylonitrile/polybutadiene copolymer, Sb.sub.2 O.sub.3 and a halogenated aromatic compound.
U.S. Pat. No. 4,162,276 which discloses polyurethane elastomer compositions with non-drip flame-retardant properties; the compositions comprise hexa-alkoxymethyl-melamine, organic halogen compounds and phosphorus substituted phosphocyclopentene oxide.
The use of flame retardant additives does not necessarily provide a satisfactory solution to the problem of flaming particle dripping. Moreover, the addition of flame retardants at the typically large quantities heretofore used in the polyurethane art, especially for foam applications, to achieve the desired flammability characteristics would be expected to adversely affect the modulus of a polyurethane elastomer. it is believed that many of the flame retardant additives commonly known and used exert a plasticizing effect when added to resins in the large amounts typically required to adequately flame retard the polyurethane product. Such an effect can be especially deleterious to the modulus of a high modulus RIM polyurethane elastomer.
Thus, despite the widespread effort to impart flammability resistance characteristics to polyurethane products to meet industry flammability standards, there still remains a need to provide high modulus polyurethane elastomers which do not drip flaming particles when exposed to a flame and which burn at a sufficiently slow rate to satisfy existing vertical burn test criteria, and yet maintain satisfactory processibility in the RIM technique and have satisfactory physical properties.